Page 124 - Chemistry ICSE Class IX

112 ICSE Chemistry – 9
Ans. From the data given,
Mass number of X = 6 + 6 = 12
Mass number of Y = 6 + 8 = 14

Both the elements contain equal number of protons (i.e., the same atomic number) but different mass numbers.
Thus, X and Y are isotopes.
Q.3. Why do noble gases exist as monatomic molecules?
Ans. Noble gases have stable electronic con gurations. Atoms of these elements cannot lose or gain or share electrons
with other atoms. As a result, noble gases exist as monatomic molecules, i.e., the noble gases exist in nature as
stable free atoms.
Q.4. Which in the following pairs is more stable?
–
+
(a) Ca, Ca or Ca 2+ (b) O, O or O 2–
Ans. (a) Ca has an electronic distribution of 2, 8, 8, 2. Thus, Ca atom has two valence electrons. The nearest noble gas
con guration is 2, 8, 8. So, Ca atom gains stability by losing 2 electrons, i.e., when Ca atom loses two electrons
2+
2+
+
2+
then Ca so obtained will be very stable. So, out of Ca, Ca and Ca , the Ca is more stable.
(b) Oxygen atom has an electronic con guration 2, 6. The nearest noble gas con guration is 2, 8. Thus, oxygen
2–
atom gains maximum stability by gaining two electrons. Therefore O is more stable.
+
Q.5. Which noble gas has an electronic con guration like that of Li ?
+
Ans. Li has a con guration of (K = 2). The noble gas having 2 electrons in its outermost shell (K-shell) is helium (He).
ASSIGNMENT 4

Isotopes, Chemical reactivity and electronic configuration, Octet rule, Electrovalent bond, Covalent bond
ϭ͘ tŚŝĐŚ ŝƐŽƚŽƉĞ ŽĨ ĐĂƌďŽŶ ŝƐ ƌĂĚŝŽĂĐƟǀĞ͍
Ϯ͘ tƌŝƚĞ ƚŚĞ ĞůĞĐƚƌŽŶŝĐ ĚŝĂŐƌĂŵ ŽĨ Ăůů ƚŚĞ ŝƐŽƚŽƉĞƐ ŽĨ ĐŚůŽƌŝŶĞ͘
ϯ͘ tŚĂƚ ĚĞƚĞƌŵŝŶĞƐ ƚŚĞ ĐŚĞŵŝĐĂů ƌĞĂĐƟǀŝƚLJ ŽĨ ĂŶ ĞůĞŵĞŶƚ͍
ϰ͘ EĂŵĞ ƚŚĞ ĞůĞŵĞŶƚ ǁŚŽƐĞ ĂƚŽŵ ĚŽĞƐ ŶŽƚ ŚĂǀĞ Ă ƚĞŶĚĞŶĐLJ ƚŽ ĂĐŚŝĞǀĞ ĂŶ ŽĐƚĞƚ͘
ϱ͘ ^ƚĂƚĞ ƚŚĞ ŽĐƚĞƚ ƌƵůĞ͘
ϲ͘ tŚŝĐŚ ĞůĞŵĞŶƚƐ ĨŽƌŵ ĐŽǀĂůĞŶƚ ďŽŶĚƐ ĞĂƐŝůLJ͍
ϳ͘ ƌĂǁ ƚŚĞ ĞůĞĐƚƌŽŶ ĚŽƚ ĚŝĂŐƌĂŵ ĨŽƌ ƚŚĞ ĨŽƌŵĂƟŽŶ ŽĨ ŶŝƚƌŽŐĞŶ ŵŽůĞĐƵůĞ͘
ϴ͘ ,Žǁ ŝƐ ĂŶ ĞůĞĐƚƌŽǀĂůĞŶƚ ďŽŶĚ ĨŽƌŵĞĚ͍

Let us Answer Some More Questions

1. Name three basic subatomic particles which are present in an atom.
2. Name the scientists who discovered (a) electron, (b) proton, and (c) neutron.
3. Which of the two, atomic number or mass number, determines the chemical properties of an element?
4. What happens when cathode rays are subjected to a strong electrical eld?
5. For which of the particles, cathode ray particle or positive ray particle, is the e/m ratio lower?

6. What is the approximate size of the nucleus of an atom?
7. Name the particle used by Rutherford in the scattering experiment.
14
12
8. What is the nuclear composition of the two isotopes of carbon: C and C ?
6
6
9. What are valence shell and valence electrons?
10. What type of electronic con guration corresponds to chemical nonreactivity?
11. State octet rule.